Chapter 7. Nortel Networks L2/3 GbESM configuration and network integration 61
section. You can find more information about this configuration in 7.6, “Basic Layer 2 entry
topology” on page69.
7.2.2 Advanced Layer 2 configurations
These configurations add additional connections between the GbESM modules and the Core
switches, using a mesh topology. This topology includes loops on most of the VLANs used,
and therefore requires the use of Spanning Tree Protocol (STP). Testing was performed
using the original STP (802.1D) as well as Rapid Spanning Tree and Multiple Spanning Tree
(802.1w and s). You can find more information about this configuration in 7.7, “Advanced
Layer 2 topology sample configurations” on page80.
7.2.3 Layer 3 configuration - static routing
This configuration uses the Layer 3 IP routing capabilities of the Nortel GbESM. Not only are
the blade servers on differing VLANs from each other, in this configuration the connections to
the Core switches are on different VLANs from all of the blades. Thus, every packet sent to or
from the blades is routed at Layer 3.
This configuration includes a full mesh topology, connecting the each of the two GbESM
switches and the two Core switches to all of the others. STP is not needed in this design
despite the full mesh because of the use of Layer 3 routing.
High Availability is achieved in this design through the use of Virtual Router Redundancy
Protocol (VRRP) and the Hot Standby option. VRRP allows the two GbESM modules to back
each other up, so that the failure of one GbESM can be survived. Hot Standby enhances the
capabilities of VRRP by allowing the backup switch to take over in the event the primary
switch is cut off from its upstream neighbors.
One key issue with Layer 3 designs is the need for L3 routers to know where to forward traffic
to a given destination. In this design, both the Core switches and the GbESM switches are
explicitly configured with the IP addresses of their neighbors. This approach is referred to as
static routing, and does not itself react to changes in the network topology. Topology
changes such as IP address reassignments, introduction of additional devices or networks,
and others could require configuration changes on all of the switches in our test environment.
You can find more information about this configuration in 7.8, “Layer 3 topology sample
configurations” on page108.
7.2.4 Layer 3 configurations - dynamic routing
These configurations are similar to the ones in the previous section but they use dynamic
routing protocols instead of static routing. These protocols enable the GbESM modules and
Core switches to provide each other with up to date information about which IP subnets they
can reach, allowing traffic to be forwarded on the optimal path. In addition, these dynamic
protocols react quickly to changes in network topology and do not require that every switch
have its configuration updated when the network changes.
These configurations include the use of VRRP — without hot-standby — on the portion of the
network connecting directly to the server blades. Hot standby can be used in a configuration
such as this if desired.
The decision to use static or dynamic routing is made by network architects in most cases.
These configurations are provided to show that the GbESM switch modules can participate in
RIP or OSPF networks if it is decided that this is the best way to integrate them with the